Effect of grinding medium characteristics of vibration mill on superfine grinding of wheat bran

A grinding medium can be generally recognized to determine the superfine impact crushing performance of wheat bran in a vibration mill. Specifically, the impact contact between wheat bran and grinding medium depends mainly on the contact form, spatial position, and filling state of the grinding medium. This study aims to explore the effect of grinding medium characteristics (such as density, size, and shape) on the impact superfine grinding performance of wheat bran in a small vibration mill. A vibration grinding experiment platform was also built with the temperature control system. A specific experimental scheme was designed for the low-temperature circulating pump and pipe fittings, according to the equivalent vibration of the vibration mill. A series of vibration impact grinding experiments of wheat bran were carried out, where the mass fraction of wheat bran powder was selected as the yield index, while the characteristic parameters of wheat bran and the particle size distribution of micro-powder were selected as the quality index. The results showed that there was a very significant effect of grinding medium characteristics on the superfine grinding of wheat bran. Specifically, the maximum yield difference rates of wheat-bran superfine powder were 50.05% caused by the density, 49.64% by the size, and 70.07% by the shape, of the grinding medium. In the case of the same masses of grinding medium with the different densities, the yields of wheat-bran coarse and fine powder were higher for the grinding medium with a high density than those with a low density, whereas, the yields of micro and superfine powder were lower than those with a low density. It demonstrated that the wheat-bran micro powder of grinding medium with a low density presented a better quality than that with a high density. Consequently, the grinding medium with a low density can be widely expected to serve as a better selection for a higher yield of wheat-bran superfine powder. Furthermore, the yields of wheat-bran coarse and fine powder firstly increased and then decreased, with the increase of grinding medium size, whereas, the yields of micro and superfine powder decreased firstly and then increased. More importantly, the quality of wheat-bran micro powder decreased throughout the process, indicating a typical quadratic nonlinear. As such, there was a high demand for the grading of grinding medium to implement the graded progressive grinding, in order to improve the grinding performance of grinding medium with different sizes. In addition, the quality of wheat-bran micro powder with the cylindrical and spherical grinding medium firstly improved and then decreased, with the increase of grinding time. Meanwhile, there was an approximately linear increase in the yield of superfine powder, where the cylindrical grinding medium was always higher than that of the spherical one. Nevertheless, there was a much higher deterioration rate of wheat-bran micro powder with the spherical grinding medium, compared with the cylindrical one. Fortunately, the quality difference of wheat-bran micro powder between the cylindrical and spherical grinding medium was less than 2.26%, when the grinding time reached 1.0 h. Consequently, the cylindrical shape of the grinding medium was preferred to improve the yield of wheat-bran superfine powder. The grinding characteristics can also be expected to significantly change the superfine grinding performance of wheat bran. This finding can greatly contribute to the potential application for the better performance of superfine grinding in a vibration mill, particularly for the industrialized large-scale production of wheat-bran superfine powder.